Math.easeInOutQuad = function (t, b, c, d) {
  t /= d / 2
  if (t < 1) {
      return c / 2 * t * t + b
  }
  t--
  return -c / 2 * (t * (t - 2) - 1) + b
}

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
var requestAnimFrame = (function () {
  return window.requestAnimationFrame || window.webkitRequestAnimationFrame || window.mozRequestAnimationFrame || function (callback) {
      window.setTimeout(callback, 1000 / 60)
  }
})()

/**
* Because it's so fucking difficult to detect the scrolling element, just move them all
* @param {number} amount
*/
function move(amount) {
  document.documentElement.scrollTop = amount
  document.body.parentNode.scrollTop = amount
  document.body.scrollTop = amount
}

function position() {
  return document.documentElement.scrollTop || document.body.parentNode.scrollTop || document.body.scrollTop
}

/**
* @param {number} to
* @param {number} duration
* @param {Function} callback
*/
export function scrollTo(to, duration, callback) {
  const start = position()
  const change = to - start
  const increment = 20
  let currentTime = 0
  duration = (typeof (duration) === 'undefined') ? 500 : duration
  var animateScroll = function () {
      // increment the time
      currentTime += increment
      // find the value with the quadratic in-out easing function
      var val = Math.easeInOutQuad(currentTime, start, change, duration)
      // move the document.body
      move(val)
      // do the animation unless its over
      if (currentTime < duration) {
          requestAnimFrame(animateScroll)
      } else {
          if (callback && typeof (callback) === 'function') {
              // the animation is done so lets callback
              callback()
          }
      }
  }
  animateScroll()
}
